A. Field of the Invention
This invention relates to systems and method for modulating and demodulating electromagnetic carriers for the transmission of binary digital data, and more particularly to such a system and method which employ a combination of frequency and phase shift keying.
B. Description of the Prior Art
The four basic binary modulation-demodulation techniques generally used for communicating binary digital data are:
CPSK--coherent phase shift keying PA1 DCPSK--differentially coherent phase shift keying PA1 CFSK--coherent frequency shift keying PA1 NCFSK--non-coherent frequency shift keying
The CPSK technique associates a binary "0" with the transmission of a carrier with some reference phase during the bit period. A binary "1" is associated with the transmission of a carrier with the phase shifted by 180.degree. from the reference. The demodulation process is accomplished by deriving a phase reference at the receiver and comparing the phase of the received carrier with the reference during each bit period to determine whether a "0" or a "1" was transmitted.
The DCPSK technique associates a "0" with no change in carrier phase from the previous bit period. A "1" is associated with a change of 180.degree. in carrier phase from the previous bit period. The demodulation process compares the phase of the carrier during the present bit period with what it was during the previous bit period and thereby establishes whether a "0" or "1" was transmitted.
The CFSK and NCFSK techniques associate a "0" with the transmission of a "space" frequency and a "1" with the transmission of a "mark" frequency. The demodulation processes for both techniques involve the identification of the received frequency as either a "mark" or "space" frequency during each bit period. The CFSK demodulation process assumes that the transmitted carrier phase can be and is modeled at the receiver and the phase information obtained in this way is used in the demodulation process to achieve improved performance. The NCFSK demodulation process assumes that the transmitted carrier phase cannot be modeled at the receiver and the frequency identification is therefore made without benefit of phase information.
When the propagation path is relatively stable, the CPSK technique offers the best performance. However, if the presence of multipath causes the phase of the received carrier to be unreliable, the performance of NCFSK is superior in performance to the other modulation systems described above because it is independent of the phase of the received signal. Accordingly, in order that a communication system perform relatively well in both high and low multipath environments, means could be employed to use CPSK modulation and demodulation during periods of high phase stability, and to use NCFSK during periods of low phase stability. By combining an NCFSK system and a CPSK system, the transmitting unit and receiving unit could be switched, in synchronism, from one mode of operation to the other to utilize whichever mode functioned best at the particular moment. However, a simple combination of such prior art systems would require that a signal be sent from the receiving terminal to the transmitting terminal to periodically provide information as to which modulation mode was preferable.